Field Guide · hardware

Also known as: Airspy HF+, Airspy HF plus, HF+ Discovery

Airspy HF+ is a software-defined radio optimised for the HF and low-VHF bands, with very high dynamic range for receiving shortwave and weak low-band signals.1 Where the VHF/UHF Airspy R2 chases wide bandwidth, the HF+ chases the opposite goal: a narrow, exceptionally clean window on a crowded low band where a single strong broadcaster can otherwise desensitise the whole receiver.

0100 MHz200 MHz300 MHz HF low VHF (60–260 MHz) Two coverage windows; a narrow, very clean capture in each
The Airspy HF+ covers HF (to ~31 MHz) and a low-VHF window (60–260 MHz), trading wide bandwidth for exceptional dynamic range.

How it works

HF is a hostile band for a receiver: hundreds of powerful shortwave, broadcast-AM and amateur signals arrive at once, and the challenge is not sensitivity but not being overloaded by the strongest of them. The HF+ is built around that problem rather than around raw bandwidth:

  • A polyphase harmonic-rejection mixer and a very selective front-end tuner suppress the images and harmonics that a simpler mixer would fold into the passband, so nearby strong signals do not create spurious responses.
  • A high-resolution sigma-delta ADC and heavy oversampling deliver very high dynamic range — the headroom to hear a weak signal sitting next to a broadcaster tens of dB louder.
  • The delivered capture is deliberately narrow (up to ~768 kHz for the HF+ Discovery), which is all a shortwave, marine or amateur channel needs, and keeps the in-band energy — and thus the demand on the ADC — small.2

The tradeoff is explicit: the HF+ will not give you the 10 MHz-wide slice an Airspy R2 does, because on HF that width would let in more strong signals than any front end could handle cleanly. It is receive-only.

Variants

  • Airspy HF+ — the original, covering HF and a low-VHF window (60–260 MHz).
  • Airspy HF+ Discovery — a smaller, refined revision with an improved preselector and even better close-in dynamic range; the current mainstream HF+ model.

Both target the same job. Against the alternatives, a basic RTL-SDR needs an upconverter or a lossy direct-sampling hack to reach HF at all and has far less dynamic range; an SDRplay RSP1A is a wideband 14-bit rival that covers HF continuously through GHz but is generally beaten by the HF+ on pure close-in HF dynamic range; and the older FUNcube Dongle is a narrowband receiver aimed more at satellite work.

Relevance to SDR

The HF+ is the receiver of choice when HF or the low VHF band is your target — shortwave broadcast, amateur SSB/CW, marine HF, ionospheric propagation study, or low-band utility monitoring — precisely because its dynamic range survives the crowded HF environment where cheaper receivers fold under overload.

For GopherTrunk this is more of a complementary tool than a core one: the land-mobile trunking systems GopherTrunk decodes (P25, DMR, NXDN, TETRA) live in VHF/UHF, where an Airspy R2 or an RTL-SDR is the right radio. GopherTrunk can drive the HF+ as a receiver, but its wideband-channelizer strengths don’t apply to the HF+’s narrow captures. Choose the HF+ when the low bands themselves are what you want to hear.

Sources

  1. Software-defined radio — Wikipedia, for background on the Airspy HF+ and HF-optimised, high-dynamic-range SDR receivers. ↩

  2. Airspy HF+ Discovery — Airspy, on the HF+ architecture, polyphase harmonic-rejection mixer, coverage and dynamic-range design. ↩

See also

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